Sealable, peelable film comprising a block copolymer peelable core layer

ABSTRACT

The present invention concerns a sealed package formed from a polymer film comprising a block copolymeric substrate and a heat sealable coating and/or skin layer on the substrate, the package being formed by wrapping the film around an article to be packaged in a manner to obtain at least one region of film overlap, and heat sealing the resulting overlapped film sections to each other to provide at least one sealed region of the package, the sealed region being subsequently openable by manually separating the overlapped film sections, effective without substantial tearing of the film at or around the sealed region.

This application is a continuation application of U.S. patentapplication Ser. No. 12/299,797, filed Nov. 25, 2008, which is a 371 ofPCT/GB2007/050215, filed Apr. 26, 2007, which claims priority of GB0609940.2, filed May 19, 2006. The entirety of all of the aforementionedapplications is incorporated herein by reference.

FIELD

The present invention relates to a sealable, peelable film and to amanufacturing method for such a film.

BACKGROUND

WO-A-96/04178 discloses a peelable film comprising a core layercomprising an olefin polymer and a heat-sealable layer comprising ablend of low density polyethylene (LDPE) and a material incompatiblewith the LDPE, such as an olefin polymer or a co- or terpolymer ofethylene, propylene or butylene. The film structure can be heat-sealedto a plastic container to form the lid of the container, or to itself toform a package.

EP-A-746468 describes a multi layer film which peels by rupture withinwhat is referred to in the specification as an intermediate layerbetween a base layer and a heat sealable layer. The intermediate layeris said to comprise preferably a blend of polymers which do notco-crystallise.

U.S. Pat. No. 5,443,915 discloses an oriented polyolefin film having awhite-opaque cold seal-receptive skin layer on one side of a core layerand a vacuum deposited metal layer on the other side of the core layer.

U.S. Pat. No. 5,358,792 discloses a heat-sealable composition comprising(a) from about 30 to about 70 wt % of a low-melting polymer comprising avery low density ethylene-based copolymer defined by a density in therange of about 0.88 g/cm3 to about 0.915 g/cm3, a melt index in therange of about 1.5 dg/min to about 7.5 dg/min, a molecular weightdistribution (Mw/Mn) no greater than about 3.5 and (b) from about 70 toabout 30 wt % of a propylene-based polymer.

U.S. Pat. No. 5,482,780 discloses an oriented polymeric α-olefin filmhaving: an isotactic propylene homopolymer core; a cold seal releaseskin layer adherent to one side of the core; and a surface treated coldseal receptive layer or such treated layer with a cold seal cohesivecomposition over the surface treatment of said layer on the other sideof the core. The cold seal release skin layer comprises a slip agent anda blend of two polymers, namely, an ethylene-propylene random copolymercontaining about 2% to 8% of ethylene in such copolymer and anethylene-butylene copolymer containing about 0.5% to 6% of ethylene insuch copolymer. The cold seal receptive layer is of anethylene-propylene random copolymer containing about 2 to 8% ofethylene.

U.S. Pat. No. 5,500,265 discloses a peelable film comprising (a) a corelayer comprising an olefin polymer, (b) a skin layer on at least onesurface of the core layer, the skin layer comprising: a blend of abutylene polymer with another olefin polymer or a polymer of butyleneand at least one other olefin, and (c) a coating layer on the skinlayer.

U.S. Pat. No. 5,716,698 relates to a peelable, oriented, opaque,multilayer polyolefin film comprising at least one opaque layer and apeelable top layer.

U.S. Pat. No. 6,248,442 relates to multilayer films that areheat-sealable over a broad temperature range. The '442 patent alsorelates to multilayer films that provide easy-opening and hermetic sealsto packages. The films of the '442 patent are made up of a core layercomprising linear low density polyethylene (LLDPE) and at least one skinlayer having a melting point of at least 10° C. below the core layermelting point.

U.S. Pat. No. 5,997,968 discloses a multilayer polyolefin filmcomprising at least three coextruded layers comprising an opaque baselayer, an intermediate layer, and an outer peelable surface layercomposed of two incompatible polyolefins, wherein the intermediate layercontains at least 80% by weight of a polyolefin having a lower meltingpoint or lower glass transition temperature than the polyolefin formingthe base layer.

U.S. Pat. No. 6,231,975 discloses a sealable film comprising an innerlayer of any olefin polymer, a sealing layer and a separable layerpositioned there between. The separable layer comprises anethylene-propylene copolymer or a blend of polyethylene and anotherolefin, which forms an incompatible mixture or blend.

Another type of peelable seal is described in our co-pending UnitedKingdom patent application 0514785.5.

There is on ongoing need for improved products in the field of sealableand peelable film structures. In particular, a need still exists for afilm structure that has a sealable outer layer that seals well to itselfor other surfaces over a broad temperature and/or pressure range andwhich is able to peel within one layer of a multi-layer film, and yetwhich maintains acceptable characteristics for the film's end use interms of at least one of gloss, haze, opacity, printability and COF. Inparticular there is a need for peelable films which can be readilyopened without creating a “z-direction” tear. In stretched polyolefinfilms, in particular biaxially orientated polypropylene films, themechanical strength of seal seams is often higher than that of the filmitself, and therefore when a sealed pack is opened is not only the sealseam which is broken apart. Typically, a tear propagates in anuncontrolled manner through the entire film upon opening. Peelable filmsare films which once sealed can be separated again mechanically withoutdamaging or destroying the film itself. Peelable seal layers aretypically required to have good sealing properties and also permitcontrolled opening of a bag or package. A peelable heat sealable filmcan be formed when a coated film is formed by coating a biaxiallyoriented polypropylene film formed as a multi layer film with a core ofa homopolymer of polypropylene with a skin layer formed on each side ormajor surface of the core with a heat sealable coating. Such skin layersmay be formed from copolymers such as copolymers of propylene andethylene including block copolymer. The skin layer formed as a melt coatduring the production of the film is believed to give a peeling sealwhen coated by virtue of incompatible phases in the melt coat. Thislimits the plain of failure to the melt coat itself, away from the corelayer avoiding any possibility of a tearing seal in the core. The resultis that the seal is weaker than a conventional seal but gives the sameseal strength throughout the pool, and doesn't tear. A z-direction teardisrupts the integrity of a multilayer film when the film is pulledapart at the seal. A film with a z-direction tear has not simplyseparated at the seal line. Instead, the separation, or tear, hasextended to other layers of the film. It is difficult to properlyreclose a package that has a z-direction tear, thereby hampering theability of the package to maintain the freshness or integrity of itscontents. When z-direction tears can be eliminated, packages are easilyrefolded and sealed by simple mechanical means, such as a clip oradhesive label.

SUMMARY

It is an object of the invention to provide a sealable polymer filmhaving peelability.

It is another object of the invention to provide a thermoplastic filmstructure with a sealable outer layer that seals well over a broadtemperature range.

A further object of the invention is to provide a thermoplastic filmstructure with a sealable outer layer that, when sealed, can be readilyopened without creating a z-direction tear.

It is a further object of the invention to provide such as films forwrapping and peelably sealing articles or products.

The present invention further provides a sealed package formed from apolymer film comprising a block copolymeric substrate and a sealablecoating and/or skin layer on the substrate.

DETAILED DESCRIPTION

It is an object of the invention to provide a sealable polymer filmhaving peelability. It is another object of the invention to provide athermoplastic film structure with a sealable outer layer that seals wellover a broad temperature range. A further object of the invention is toprovide a thermoplastic film structure with a sealable outer layer that,when sealed, can be readily opened without creating a z-direction tear.It is a further object of the invention to provide such as films forwrapping and peelably sealing articles or products.

It has now been discovered that peelable, sealable films can be obtainedin a multilayer film in which a block copolymeric component is providedin the core. It is believed, although the invention described herein isnot bound or limited by this theory, the film of the invention peels inthe core layer.

Biaxially oriented films comprising block copolymer cores are known inconnection with a number of applications. For example, WO-A-2005/033195discloses an impact copolymer film having defined water vapourtransmission rate properties which are said to be useful in foodpackaging applications. EP-A-0987290 discloses similar films for similarpurposes. Yet further food packaging films utilising the gas and vapourtransmission properties of block copolymeric films are taught inWO-A-2004/056567. However, the use of a block copolymeric material inthe core layer of a multi layer film provided with a sealable outerlayer, wherein the sealed film is rendered peelable in the blockcopolymer core has not hitherto been realised.

According to the present invention there is provided a sealable andpeelable film comprising a block copolymeric substrate and a sealablecoating and/or skin layer thereon.

The film of the invention is preferably a multilayer film comprising ablock copolymeric core layer and at least one a sealable skin layer. Theskin layer may be inherently sealable, or may be coated with a sealablecoating.

Preferably, the at least one skin layer is coated with a sealablecoating.

By “sealable” is meant heat sealable, cold sealable, pressure sealable,or any suitable combination thereof. For most applications, heatsealable films will be used.

The block copolymeric substrate or core layer preferably comprises over90%, more preferably over 95%, still more preferably over 97.5% of thetotal film thickness.

In one embodiment of the invention the core layer is composed entirelyof the block copolymeric substrate. In an alternative embodiment theblock copolymeric substrate may be blended with one or more othersuitable materials to form the core layer, provided that the core layerretains its peelable characteristic.

The core layer may be non-cavitated, or may be cavitated if an opaquefilm is desired.

Sealable and peelable film structures according to the present inventionmay also include one or more intermediate layers positioned between thecore layer comprising the peelable block copolymeric substrate and thesealable outer layer.

One advantage of the present sealable and peelable films is thatpeelability is provided in the core, allowing the manufacturer freedomto adapt the skin layer(s) and/or intermediate layer(s) and/or coatings)to meet manufacturing requirements of; for example, gloss, haze,opacity, printability, COF, etc.

The films of the invention will commonly find application in thepackaging of articles of commerce, for example in reamwrap and overwrapapplications.

According to the present invention there is a provided a sealed packageformed from a polymer film comprising a block copolymeric substrate anda sealable coating and/or skin layer on the substrate, the package beingformed by wrapping the film around an article to be packaged in a mannerto obtain at least one region of film overlap, and heat sealing theresulting overlapped film sections to each other to provide at least onesealed region of the package, the sealed region being subsequentlyopenable by manually separating the overlapped film sections, effectivewithout substantial tearing of the film at or around the sealed region.

Without wishing to be bound by the theory, it seems likely that theblock copolymeric substrate provides the desired peelability by allowingthe heat sealed film to peel away from its overlapped neighbour bypeeling off within the block copolymeric core of the film.

Preferably, the polymer film is a multilayer film comprising the blockcopolymeric substrate and at least one skin layer of a differentmaterial. The at least one skin layer may be a polymeric material, andmay comprise be homopolymeric and/or copolymeric materials, and may be ablend of two or more such materials. The skin layer may be provided onthe substrate by coextrusion, by lamination, or by any other suitablemethod. Preferably, the skin layer is a coextrudate. The skin layer mayalso be formed from a peelable material, for example from a blockcopolymeric material. Alternatively, the skin layer may be non-peelable,and may for example comprise a non-block copolymeric structure, forexample a random copolymeric structure, a homopolymeric structure or asuitable blend of materials.

The presence of a skin layer may assist in the realisation of theinvention in its more preferred aspects because peeling initiated in thecore, between two block copolymeric components for example, may beprevented or hindered from propagating beyond the core layer.Propagation of the peel through the skin layers of the film may giverise to a tearing film rather than a peelable film.

Accordingly, the invention provides in one of its preferred aspects asealed package formed from a polymer film comprising a block copolymericsubstrate and at least one skin layer on the substrate, the skin layerbeing formed from a sealable material, or otherwise being coated with asealable coating, the package being formed by wrapping the film aroundan article to be packaged in a manner to obtain at least one region offilm overlap, and sealing the resulting overlapped film sections to eachother to provide at least one sealed region of the package, the sealedregion being subsequently openable by manually separating the overlappedfilm sections, effective without substantial tearing of the film at oraround the sealed region.

Preferably the sealable material and/or the scalable coating is a heatsealable material and the overlapped film sections are sealed to eachother by heat sealing.

Preferably, the film used in forming the sealed packages of theinvention comprises a block copolymeric substrate, at least one skinlayer and a heat sealable coating on the skin layer.

Generally, for ease of manufacture, the film will comprise a blockcopolymeric core substrate sandwiched between skin layers of the samematerial, the skin layers being formed on the substrate by coextrusiontherewith, and a heat sealable coating on one surface, preferably onlyone surface, of the resulting film.

The sealed package of the invention is preferably formed by severing asheet of the polymeric film from a web of the film and wrapping thesevered polymeric film sheet around an article to form a film tube withwrapped ends overlapping and sealing the overlapped ends to form a girthseal. Preferably the tube is then closed at each end and sealed to forman envelope or parcel seal.

One particular application of the film and sealed packages of theinvention is in ream wrap.

Reams of cut paper for copy machines, computers, printers and otherapplications are most commonly packaged for shipping, storage and retailsale in ream wrappers made of various wrap materials. These wrapmaterials have traditionally been paper, plastic film, or apaper/plastic film laminate.

The wrap materials protect the wrapped paper product from physicaldamage and moisture pick up during slippage and storage. The wrapmaterials also protect the wrapped product from physical damage duringrepeated handling and stocking on retail shelves.

The distribution of reams of papers has changed from boxes for largerusers to wrapped reams for sale in individual packages containing e.g.500 sheets. Such distribution of reams of papers has placed increasingdemands on the wrapper due to more frequent handling of the individualreams. Increased handling of the reams has resulted in more reamsbreaking open, damaging the wrapped paper product by allowing to pick upmoisture, tear or get minor curl—physical damage that ultimately resultsin jams in the end—user's printer or copy machine. However the resultantability of a package to avoid damage arising from handling means thatthe packaged product is more difficult to open.

One way of overcoming this problem would be to provide sealing meansthat possess a mechanical resistance sufficient to maintain the sealintact during storage and transport until such time as the packaging orwrapping is to be opened but the package remains capable of being openedmanually at or near the seal without risk of damaging the wrapped paperproduct.

The invention is particularly applicable in the field of ream wraptechnology. Thus, according to the invention there is provided a sealedream wrap package in which the packaged article is a stack of paper.

The films of the invention are also especially applicable in the fieldof overwrap.

The block copolymeric substrate preferably comprises at least oneolefinic component. More preferably, at least two olefinic componentsare present, the block copolymer being at least partially formed by theblock copolymerisation of one olefinic component, or mixture of olefiniccomponents, with another olefinic component, or mixture of olefiniccomponents. For example, the block copolymeric substrate may comprisepolypropylene and polyethylene.

Also provided in accordance with the present invention is a sealedpackage comprising polymer film in accordance with the above printed onits skin layer with at least one ink.

The film or the skin layer of the film may comprise additional materialssuch as anti-block additives, opacifiers, fillers, UV absorbers,cross-linkers, colourants, waxes and the like.

The film of the invention may be further treated, by corona dischargetreating for example, further to improve ink receptivity of the film orof the skin layer of the film.

The film substrate may comprise a blend of the block copolymericsubstrate with another polymeric material, for example polyethylene,polypropylene, mixtures thereof, and/or other known polyolefins.

The polymeric film can be made by any process known in the art,including, but not limited to, cast sheet, cast film, or blown film.This invention may be particularly applicable to films comprisingcavitated or non-cavitated polypropylene films, with a block copolymerpolypropylene/polyethylene core and skin layers with a thicknesssubstantially below that of the core layer and formed for example fromrandom co-polymers of ethylene and propylene or random terpolymers ofpropylene, ethylene and butylene. The film may comprise a biaxiallyorientated polypropylene (BOPP) film, which may be prepared as balancedfilms using substantially equal machine direction and transversedirection stretch ratios, or can be unbalanced, where the film issignificantly more orientated in one direction (MD or TD). Sequentialstretching can be used, in which heated rollers effect stretching of thefilm in the machine direction and a stenter oven is thereafter used toeffect stretching in the transverse direction. Alternatively,simultaneous stretching, for example, using the so-called bubbleprocess, or simultaneous draw stenter stretching may be used.

The films used in accordance with the present invention can be of avariety of thicknesses according to the application requirements. Forexample they can be from about 10 to about 240 μm thick and preferablyfrom about 50 to about 90 μm thick.

In a multi-layer film in accordance with the invention having at least asubstrate later and a skin layer, the skin layer is preferably inkprintable. The skin layer has a thickness of from about 0.05 μm to about2 μm, preferably from about 0.1 μm to about 1.5 μm, more preferably fromabout 0.2 μm to about 1.25 μm, most preferably from about 0.3 μm toabout 0.9 μm.

The invention will now be more particularly described with reference tothe following examples.

Example 1 Preparation of Film

A three layer polymeric tube was formed by coextruding a core layer (ofpolypropylene homopolymer in the Comparative Example, and of apolypropylene-polyethylene block copolymer in the Example according tothe invention) with a layer of polyethylene/polypropylene/polybutyleneterpolymer (a random copolymer) as a skin layer on one side of the corelayer, and with a blend of polypropylene/polyethylene andpolypropylene/polybutylene random copolymers as a lamination layer onthe other side of the core layer. The tube was cooled and subsequentlyreheated before being blown to produce a three layer biaxially orientedfilm tube. The film tube was then nipped and laminated to form alaminated film having an inner lamination layer of approximately 0.7 μmthickness, a core layer of approximately 28 to 29 μm thickness on eitherside of the lamination layer, and a random terpolymer skin layer ofapproximately 0.3 μm thickness.

The laminated film was then primed with a polyethylene-imine primer andcoated on one side with an acrylic coating. The acrylic coating wasformulated as a production batch using as a source of the acrylicmaterial present in the coating WB1240. WB1240 is an acrylic copolymerdispersion in water supplied by UCB Surface Specialties of Rued'Anderlecht 33 B-1620 Drogenbos Belgium and the quantity used was thatnecessary to give a coating composition on the film in which the WB1240formed 92.5 wt % of the entire coating. The coating composition alsocontained 0.25 wt % of polymethylmethacrylate particles as antiblock and7.5 wt % of Carnauba wax.

The uncoated laminated film compositions are presented in Table I:

TABLE 1 Film Core Inner Outer Example 1 Block Lamination Polymer RandomCopolymer (PP/PE, PP/PB Blend) Terpolymer (PP/PE) (PP/PE/PB) ComparativeHomopolymer Lamination Polymer Random Example 1a (PP) (PP/PE, PP/PBBlend) Terpolymer (PP/PE/PB)

Both films exhibited satisfactory characteristics for ream wrapapplication with respect to their gloss, wide and near angle haze, Ctadhesion, blocking side and COF measurements, all of which wereconducted in accordance with standard procedure.

The coated films were then heat sealed around a paper stack to form asealed ream wrap package. The film seals were found to be tear seals inthe Comparative Example, and peel seals in the Example in accordancewith the invention.

1. A sealable and peelable film consisting of a block copolymericpeelable core layer consisting of over 95% of the film thickness, and asealable, non-peelable skin layer wherein said skin layer has athickness from about 0.1 μm to about 1.5 μm comprising a non-blockcopolymeric structure.
 2. The film according to claim 1, wherein theskin layer is heat sealable.
 3. The film according to claim 1, whereinthe block copolymeric peelable core layer consists of over 97.5% of thefilm thickness.
 4. The film according to claim 1 in the form of apackage.
 5. The film according to claim wherein the block copolymericcore comprises at least one olefinic component.
 6. The film according toclaim 5, wherein the block copolymeric peelable core layer comprises atleast two olefinic components, the block copolymer being at leastpartially formed by the block copolymerisation of one olefiniccomponent, or mixture of the olefinic components, with another olefiniccomponent, or mixture of olefinic components.
 7. The film according toclaim 6, wherein the block copolymeric peelable core layer comprisespolypropylene and polyethylene.
 8. A sealed package formed from apolymer film consisting of a block copolymeric peelable core layerconsisting of over 95% of the film thickness, and a sealable,non-peelable skin layer on the core layer, wherein said skin layer has athickness from about 0.1 μm to about 1.5 μm comprising a non-blockcopolymeric structure; the package being formed by wrapping the filmaround an article to be packaged in a manner to obtain at least oneregion of film overlap, and heat sealing the resulting overlapped filmsections to each other to provide at least one sealed region of thepackage, the sealed region being openable by manually separating theoverlapped film sections, effective without creating a z-tearing of thefilm at or around the sealed region.
 9. The sealed package according toclaim 8, formed by severing a sheet of the polymeric film from a web ofthe film and wrapping the severed polymeric film sheet around an articleto form a film tube with wrapped ends overlapping and sealing theoverlapped ends to form a girth seal wherein the tube is then closed ateach end and sealed to form an envelope or parcel seal.
 10. The sealedpackage according to claim 8, wherein the block copolymeric peelablecore layer comprises at least one olefinic component.
 11. The sealedpackage according to claim 10, wherein the block copolymeric peelablecore layer comprises at least two olefinic components, the blockcopolymer being at least partially formed by the block copolymerizationof one olefinic component, or mixture of olefinic components, withanother olefinic component, or mixture of olefinic components.
 12. Thesealed package according to claim 11, wherein the block copolymericpeelable core layer comprises polypropylene and polyethylene.
 13. Asealed package formed from a polymer film consisting of a blockcopolymeric peelable core layer consisting of over 95% of the filmthickness, and one skin layer on the core layer, the skin layer beingformed from a non-peelable material coated with a sealable coating,wherein said skin layer has a thickness from about 0.1 μm to about 1.5μm comprising a non-block copolymeric structure, the package beingformed by wrapping the film around an article to be packaged in a mannerto obtain at least one region of film overlap, and sealing the resultingoverlapped film sections to each other to provide at least one sealedregion of the package, the sealed region being openable by manuallyseparating the overlapped film sections, effective without creating az-tearing of the film at or around the sealed region.
 14. The sealedpackage according to claim 13, wherein the sealable coating is a heatsealable coating and the overlapped film sections are sealed to eachother by heat sealing.
 15. The sealed package according to claim 13,wherein the sealed package is a ream wrap package.
 16. The sealedpackage according to claim 13, wherein the sealed package is an ove rappackage.